High strength stainless steels



July 5, 1966 K. M. CARLSEN 3,259,528

HIGH STRENGTH STAINLESS STEELS Filed May 2, 1962 "/0 CARBON 2 3 28432B4l 2842 X x x x e Q Q 2835 2836 2837 2838 2840 2844 l I l l l T l l lI 3.0 4.0 5.0 6.0 ADJUSTED NICKEL IN'ALLOY Flg. l.

320 U) a. o o 9 300 284 m 2 0 28 3 8 gem o 6 2 43 x2846 l l l l 3.0 4.05.0 6.0

ADJUSTED NICKEL IN ALLOY INVENTOR.

KURT M. CARLSEN F' .2. BY

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his ATTORNEY United States Patent 3,259,528 HIGH STRENGTH STAlNLESSSTEELS Kurt M. Carlsen, Bergen, Norway, assignor to Jones &

Laughlin Steel Corporation, Pittsburgh, Pa., 21 corporation ofPennsylvania Filed May 2, 1962, Ser. No. 191,889 2 Claims. (Cl. 14837)This invention relates to stainless steels which are austenitic in theannealed condition. It is more particularly concerned with steels ofthat type in which the preponderance of the austenite is transformed tomartensite by cold-Working so as to raise the yield and tensilestrengths of the steels to extremely high values.

A standard grade of austenitic steel is AISI type 301, which is alsoknown as 17-7 because its nominal chromium content is 17% and itsnominal nickel content is 7%. The section entitled Wrought StainlessSteels of Metals Handbook for 1961, published by The American Societyfor Metals, lists on page 416 the properties of type 301 stainless steelin the annealed and in the cold-worked conditions. Yield strength at .2%offset of conventional 301 type steel is about 33,000 p.s.i., which istypical of an austenitic steel. However, cold-working this grade greatlyincreases its mechanical properties. If the steel is cold-reduced about45% to 50%, its yield strength is raised to as much as 200,000 p.s.i.The cold work transforms a portion of the austenite into martensite, andit is considered that the pronounced strengthening is caused bymartensite precipitating along the slip planes of the alloy.

While it is possible to treat certain known austenitic stainless steelso as to obtain yield strengths somewhat higher than 200,000 p.s.i.,that high performance either requires expensive modifications of thesteel composition, or expensive treatments. It would be desirable tohave available a steel no more complicated chemically than type 301, buthaving a yield strength on the order of 250,000 to 300,000 p.s.i.

Accordingly, it is an object of my invention to provide a simplechromium-nickel stainless steel having high yield and tensile strengthin the cold-worked condition. It is another object to provide such asteel which has normal properties in the annealed condition. Otherobjects will appear in the course of the following description of myinvention.

I have found that very high yield and tensile strengths are exhibited bycold-reduced steels of a composition similar to that of type 301 if thecarbon and nickel contents are adjusted to each other in a manner to bedescribed. In the accompanying drawings, FIGURE 1 illustratesgraphically the relation between carbon and nickel in steels of myinvention, and FIGURE 2 illustrates the effect of changes in nickelcontent on the yield strength of a steel of my invention.

The steels of my invention generally contain nickel from about 3.5% toabout 6.6% and carbon from about .05% to about .15%, the nickel contentbeing in the lower end of its range when the carbon content is in thehigher end of its range, and vice versa, so that those elements may besaid to be adjusted to be inversely proportional to each other. Thesteel contains manganese in the amounts generally found in austeniticstainless steels of the A181 300 series, that is, not more than 2% andpreferably in the range from about 90% to about 1.10%. Silicon islikewise present in amounts generally found in conventional steels ofthat series, that is, not more than about 1% and preferably betweenabout .50% and .70%. The nitrogen content ranges from about to about.10%. The metalloids phosphorus and sulphur and other incidentalimpurities are present in normal amounts. Enough chromium must bepresent to render "ice the steel austenitic in the annealed state, butthe chromium content does not exceed about 18%. The minimum chromiumcontent is about 16%.

It is known that in austenitic steels of the A181 300 series, chromiumand nickel, within limits, are interchangeable, and they areinterchangeable in my steels to some extent. Within the limits of thoseelements which I have mentioned, I find that chromium and nickel can beinterchanged in the proportion of about 1.4% chromium to 1% nickel.Because of this interchangeability, I find it convenient in theparagraphs to follow to discuss the dependence of physical properties onthe chemistry of my steel in terms of adjusted nickel or adjustedpercentage nickel. The adjusted nickel is actual nickel content of eachsteel mentioned lowered or raised an amount depending on the differencebetween the actual chromium content of the steel and the value 17.4%,the proportion of chromium to nickel being that above stated.

As the carbon content of the steels herein described is raised, and thenickel content correspondingly lowered, the steels become increasinglydifiicult to work in the annealed state, so that the practical upperlimit of carbon content is about .15 the corresponding practical lowerlimlt of nickel content being about 3.5

TABLE 1 HeatNo. 0 Mn P 8 81 Cr Ni Ni N *Adjusted.

In Table 1 are listed the designations and compositions of a number ofheats of steel of my invention, as well as several heats outside thescope of my invention. In Table 2 are listed the pertinent physicalproperties of the steels of Table 1, both in the annealed state andafter being cold reduced 55%. FIGURE 1 is a plot of carbon contentagainst adjusted nickel content for those steels. Steels of myinvention, which are identified by circles, fall within the areaa-b-c-d-e marked out on FIGURE 1. Steels outside the scope of myinvention are indicated by crosses. It will be understood by thoseskilled in the art that the lines marking out the area abcd-e in FIGURE1 cannot be precisely located, because a physical property such as theyield strength of steel can be measured only with a fair degree ofreproducibility. Furthermore, the change of yield strength with changeof composition of the steel is not abrupt, as may be observed fromFIGURE 2. That figure is a plot of yield strength against adjustednickel content for steels of Tables 1 and 2 containing .07% carbon, andcold worked by reducing them 55%.

Table 2 shows that the steels falling within the area abcde of FIGURE 1had yield strengths in the annealed condition of less than about 40,000p.s.i., that is to say, the steels were all austenitic. The same steelsafter being cold-reduced about 50% all exhibited yield Patented July 5,1966 3 strengths greater than about 250,000 p.s.i. The steels fallingoutside the area a-b-c-d-e either had yield strengths higher than about40,000 p.s.i. in the annealed condition, thus showing that they were notaustenitic,

4 I claim: 1. A cold reduced austenitic-martensitic steel having a yieldstrength not less than about 250,000 p.s.i., said martensite beingproduced by cold reducing the steel about or yield strengths lower than250,000 p.s.i. in the cold- 5 55%, consisting of about 3.5 to about 6.5%nickel, about worked condition, or both. .05 to about .15% carbon, notmore than 2% manganese, FIGURE 2 illustrates the high strengths obtainedfrom not more than 1% silicon, sufilcient chromium to render an .07%carbon steel if its nickel content is critically the steel austenitic inthe annealed condition but not more adjusted to its carbon content inaccordance with my than about 18%, and the balance iron and incidentaliminvention. The steels plotted in FIGURE 2 which are purities in normalamounts, the carbon content and the austenitic in the annealed conditionare indicated by adjusted nickel content, adjusted to a chromium contentcircles, While those which are not austenit-ic in the anof 17.4% on thebasis of 1% nickel being equivalent to nealed condition are indicated bycrosses. That figure 1.4% chromium, defining a point falling Within thearea shows that an .07% carbon steel displays maximum yield abcde ofFIGURE 1. strength at an adjusted nickel content of about 5.4%. 2. Asteel as in claim 1, in which the carbon and ad- That yield strength isin the neighborhood of 275,000 justed nickel contents are related toeach other by the p.s.i. for the cold-worked steel, but in the annealedstate equation: the steel has a yield strength of less than about 40,000P 4 ercent n1ckel=8.3%-31 (Percent carbon p.s.1. FIGURE 2 shows that they1e1d strength of the steel in the cold-worked condition drops off asthe ad- 0 References Cited by the Examiner justed nickel content isincreased or reduced from its UNITED STATES PATENTS critical value forthe given carbon content. The yield strength of the steel in theannealed condition likewise 3,903,386 9/1959 WaXWelleI' 3 8 increases asthe adjusted nickel content is reduced from OTHER REFERENCES itscritical value. Similar curves could be plotted for steels of othercarbon contents within the range of my Stamless f Pagfis 210 and Ed 1t9dby invention. An empirically derived equation relating car- Zapfie-Published In 1949 by the American Soclety for bon and nickel for maximumyield strength in steels of Metals: Cleveland: OblomY 13: HYLAND BIZOT,Primary Examiner.

Percent (Percent DAVID L. RECK, Examiner.

That equation is plotted as a broken line in FIGURE 1. P. WEINSTEIN,Assistant Examiner.

TABLE 2 Annealed Cold Rolled Heat N o. 0.2% Ultimate Elonga- 0.2%Ultimate Elonga- Yield Tensile tion, Yield Tensile tion, Strength,Strength, Percent Strength, Strength, Percent p.s.i. p.s.i. p.s.i.p.s.i.

1. A COOL REDUCED AUSTENITIC-MARTENSITIC STEEL HAVING A YIELD STRENGTHNOT LESS THAN ABOUT 250,000 P.S.I., SAID MARTENSITE BEING PRODUCED BYCOLD REDUCING THE STEEL ABOUT 55%, CONSISTING OF ABOUT 3.5 TO ABOUT 6.5%NICKEL, ABOUT .05 TO ABOUT .15% CARBON, NOT MORE THAN 2% MANGANESE, NOTMORE THAN 1% SILICON, SUFFICIENT CHROMIUM TO RENDER THE STEEL AUSTENITICIN THE ANNEALED CONDITION BUT NOT MORE THAN ABOUT 18%, AND THE BALANCEIRON AND INCIDENTAL IMPURITIES IN NORMAL AMOUNTS, THE CARBON CONTENT ANDTHE ADJUSTED NICKEL CONTENT, ADJSTED TO A CHROMIUM CONTENT OF 17.4% ONTHE BASIS OF 1% NICKEL BEING EQUIVALENT TO 1.4% CHROMIUM, DEFINING APOINT FALLING WITHIN THE AREA A-B-C-D-E OF FIGURE 1.